This application requests support to continue research aimed at understanding peripheral and central factors in auditory masking both in listeners with sensorineural hearing loss and in listeners with normal hearing. The specific aims include examining the role of perceptual and cognitive factors in auditory masking, determining the role of centrally-based "informational masking" in the communication difficulties experienced by listeners with hearing loss, and extending the theory of auditory masking to include informational masking. During the past award period, substantial progress was made in the understanding of informational masking - that is, masking that cannot be attributed to overlapping patterns of excitation. One important finding from recent work is that persons with sensorineural hearing loss appear to have great difficulty in segregating and attending to auditory "streams" in complex environments. This difficulty leads to increased informational masking and may be a significant part of the communication difficulties experienced by listeners with hearing loss. The proposed research plan is intended to further examine this finding and to extend the empirical and theoretical work on auditory masking. The empirical work consists of a series of human psychophysical and speech identification experiments in which signals or targets are masked by sounds having different proportions of peripherally-based "energetic masking" and centrally-based informational masking. A fundamental aspect of the research plan is to test the effectiveness of various cues that normally facilitate perceptual segregation of sounds. In addition, the usefulness of varying degrees of a priori information is examined. These experiments span a range of tasks from detection through discrimination to nonspeech and speech identification. We propose to test groups of listeners with sensorineural hearing loss, and matched groups of listeners with normal hearing, on a set of masking experiments that assess how hearing loss and task complexity interact. The theoretical work has several facets, including developing a better understanding of the mechanisms of masking and of modeling those mechanisms in both normal-hearing and hearing-impaired listeners. The ultimate goal of this work is to understand why people with hearing loss experience such great difficulty communicating in difficult situations like restaurants, parties, or other circumstances where there are many sounds at once and the listening situation is complex and uncertain.